Continuous electrochemical process for reaction of compounds containing aliphatic unsaturation

ABSTRACT

A PREFERRED METHOD IS PROVIDED FOR THE TREATMENT OF AQUEOUS MEDIA FROM WHICH OLEFIN OXIDE HAS BEEN SEPARATED, THE OLEFIN OXIDE HAVING BEEN PRODUCED FROM AN OLEFIN VIA A HALOHYDRIN INTERMEDIATE, WHICH COMPRISES TREATING THE AQUEOUS MEDIA WITH AN INORGANIC OXIDIZING AGENT SUCH AS A HALOGEN, A METAL HYPOHALITE, HYDROGEN PEROXIDE OR OZONE-ENRICHED AIR AFTER REMOVAL OF THE OLEFIN OXIDE PRODUCT FOLLOWED BY SEPARATION OR DESTRUCTION OF COMTAMINANTS FROM THE AQUEOUS MEDIA PRIOR TO RECIRCULATION THEREOF TO AN ELECTROLYTIC CELL IN WHICH THE HALOGEN REQUIRED TO FORM THE HALOHYDRIN FROM THE OLEGIN IS GENERATED.

United States Patent CONTINUOUS ELECTROCHEMICAL PROCESS FOR REACTION OF COMPOUNDS CONTAINING ALIPHATIC UNSATURATION Joseph-Adrien M. Leduc, Short Hills, and Charles Lurie, Plainfield, N.J., and Walter Kronig and Peter Konrad, Leverkusen, Germany, assignors to Pullman Incorporated, Chicago, Ill., and Farbenfabriken Bayer Aktiengesellschaft, Leverkusen, Germany, fractional part interest to each No Drawing. Filed Mar. 24, 1969, Ser. No. 809,962

Int. Cl. B01k 1/00 U.S. Cl. 204-131 29 Claims ABSTRACT OF THE DISCLOSURE A preferred method is provided for the treatment of aqueous media from which olefin oxide has been separated, the olefin oxide having been produced from an olefin via a halohydrin intermediate, which comprises treating the aqueous media with an inorganic oxidizing agent such as a halogen, a metal hypohalite, hydrogen peroxide or ozone-enriched air after removal of the olefin oxide product followed by separation or destruction of contaminants from the aqueous media prior to recirculation thereof to an electrolytic cell in which the halogen required to form the halohydrin from the olefin is generated.

This invention relates to an improvement in the production of oxygenated derivatives of compounds having aliphatic unsaturation in a system comprising an electrolytic cell in which an aqueous medium containing a halide electrolyte is subjected to electrolyte to generate halogen at the anodic surface and hydroxyl ion at the cathodic surface. More particularly, the invention relates to a method for preventing deposition of a coating on an electrode of the cell and fouling of the electrolyte during continuous operation.

It has been found that olefin oxides may be produced by contacting an olefin with an aqueous medium in which halogen has been generated by electrolysis of an aqueous medium containing a metal halide electrolyte to form the corresponding halohydrin derivative and subsequently dehydrohalogenating the halohydrin with the aqueous alkaline medium produced at the cathode of the cell to form olefin oxide product. The alkaline medium is then treated to remove olefin oxide product and is then preferably recycled to the electrolytic cell in continuous operation. It has been found that in a prolonged continuous recycle operation, the anode of the electrolytic cell becomes coated with an impurity which interferes with the efficient operation of the cell resulting in erratic performance and requires interruption of the process to remove the adherent coating or to actually replace the anode.

It is, therefore, an object of the present invention to provide an improvement in the manufacture of olefin oxides in a system comprising an electrolytic cell in which the aqueous electrolytic medium is recycled to the cell after the olefin oxide product has been separated therefrom. A further object is to provide a process of the aforesaid type whereby the build-up of a coating of impurities on the anodic surface of the cell and fouling of the electrolyte is substantially avoided during continuous recycle operation. Another object is to provide an electrochemical process wherein product production and operating voltage in the cell remain uniform during continuous operation. Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.

In accordance with the teachings of this invention, a method is provided which comprises subjecting a liquid medium from an electrolytic cell and having a metal halide electrolyte dissolved therein to treatment with an inorganic oxidzing agent followed by passing said treated electrolyte to a separation zone for removal of contaminants from the electrolyte solution prior to recycle in the system. In a preferred aspect, a method is provided which comprises subjecting an aqueous medium contained in an electrolytic cell and having a halide electrolyte dissolved therein to the action of a direct electric current to generate halogen at the anode and hydroxyl ion at the cathode, contacting an olefinic compound with the aqueous medium thereby forming the halohydrin derivative of the olefin, dehydrohalogenating the halohydrin to olefin oxide in an alkaline medium produced at the cathode of the cell, separating olefin oxide from the aqueous medium, introducing an inorganic oxidizing agent into the aqueous medium containing a contaminant a fter separation of the olefin oxide, passing the aqeous medium after said treatment with oxidizing agent through a contaminant removal zone and recirculating the decontam inated aqueous medium to the electrolytic cell. By subjecting the aqueous medium after separation of olefin 0X- ide to the decontamination steps of this invention, it is found that the overall process may be continuously operated for prolonged on-stream periods while substantially preventing the detrimental deposition on the anode of the aforementioned coating, i.e., the integrity of the anodic surface is maintained and substantially constant operating conditions and yields of desired product are realized.

In the manufacture of olefin oxides from olefin via the halohydrin intermediate, the overall process comprises an electrolytic cell having an anodic and a cathodic surface with preferably a diaphragm therebetween and containing an aqueous solution of a metal halide which upon electrolysis generates halogen in the anolyte and hydroxyl ion in the catholyte. Olefin is contacted with anolyte comprising halogen or its hydrolysis products to form the halohydrin. The halohydrin forming reaction may be carried out by contacting the olefin with anolyte within the cell or in a contacting tower external of the cell proper or partially in both. The halohydrin containing aqueous medium is then subjected to dehydrohalogenation in order to convert the halohydrin to olefin oxide by treatment with the catholyte which is alkaline due to the cathodic reaction which results in the formation of hydroxyl ion. The dehydrohalogenation may be carried out in the alkaline catholyte contained within the cell or in a step external of the cell or partially in both. These various methods are described in further detail in US. Pat. No. 3,288,692 and Belgian Pat. No. 705,083 and No. 705,- 084, the details of which are incorporated by reference herein. The method of this invention is preferably applied with advantage to these various methods for producing olefin oxide for in each system the aforementioned difficulties brought about by the build-up of deposits on the anodic surface are observed.

The present process for prevention of coating deposits can be applied to any continuous electrochemical process wherein an unsaturated organic compound is oxidized and the electrolyte oxidation medium is withdrawn from the electrolytic cell and is recycled as at least part of the electrolyte fed to the process after the oxidation product is removed. Illustrative of processes in this category other than the preferred process described above, are the electrochemical processes for producing a glycol from an olefin or an olefin from acetylene in addition to olefin oxide from olefin as described and referred to in US. Pat. No. 1,253,617. The improvement of this invention is also applicable to methods employing a diaphragmless electrochemical cell or mercury cathode cell wherein olefin oxide is obtained from olefin in the presence of the type of electrolyte medium described, for example, in U.S. Pat. No. 3,288,692. All of these processes, when carried out in a continuous manner with recycle of electrolyte, are subject to electrode coating which gradually lowers the efficiency of the process.

Generally, the electrochemical processes to which the method of the present invention is applied, employ a cell of the diaphragmless or diaphragm type. The diaphragmless cell can be of the gravity or non-gravity type which may or may not contain partitions as cell dividers between the anode and the cathode. The cell may also contain a plurality of anode and cathode chambers as illustrated, for example, by U.S. Pat. No. 3,342,717, including the stacked cells thereof or of U.S. Pat. No. 3,288,692.

The anode of the cell comprises a solid or a porous material or a substrate having distributed thereon electrolyte-porous and electrolyte-impervious surfaces and can also contain a hollow core or an internal chamber for the introduction of the organic reactant into the electrolyte solution. The anode may be composed of graphite, platinum, platinized titanium, platinized tantalum, titanium coated with a mixture of platinum and at least one other noble metal such as iridium or rhodium, platinized iridium, magnetite, titanium, lead, or an inert substrate such as polyethylene, polypropylene, polyurethane, Teflon,

or a perfluorochloro plastic, etc. metalized with copper or silver and having platinum deposited thereon as the metal which is exposed to the electrolyte medium. Electrodes of the latter type are prepared in accordance with the techniques described in U.S. Pat. No. 3,235,473. The preferred anodes of the present invention, however, comprise -a metal substrate, most preferably titanium, coated with a noble metal or alloy of a noble metal in the form of elemental metal or metal oxide as described and referred to in U.S. Pat. No. 3,379,627.

The cathode may be composed of any conductive material which is chemically inert to caustic and is usually composed of steel, stainless steel or an amalgam such as the amalgam cells described or referred to in U.S. Pats. No. 3,394,059 and .No. 3,288,692. The cathode is more often in the form of a ferrous metal screen or expanded sheet and may be .used with or Without a diaphragm. It is to be understood, however, that bipolar cells containing bipolar electrodes having a body of conductive material interposed between an anode surface and a cathode surface can also be employed if desired. A cathode of approximately the same surface area as the anode is conveniently employed.

Diaphragms, when employed in the cell, can be any of the .known types. The diaphragm is composed of any suitable permeable or porous inert material such as, for example, asbestos, Teflon, polyethylene, polyacrylonitrile, a perfluorochloro plastic polymer, a thermoplastic material, such .as vinyl chloride polymers or copolymers, for example, copolymers with vinylidene chloride, polybutylene, polystyrenes, polypropylene, polyurethane, etc. The materials can be employed as permeable plates or films, or as fibers in the form of fabrics or as fleeces.

The preferred electrolyte which is charged to the electrolytic cell is an aqueous solution of a metal halide, chlorides and bromides being most preferred. Usually employed are the halides of sodium, potassium, lithium, barium, calcium, strontium, magnesium or mixtures thereof.

A mixed electrolyte system can be employed for improving the conductivity inasmuch as the metal hydroxide which forms at the cathode is utilized within the system and. since economics of the process does not depend upon recovery of hydroxide as a product of the process, mixed electrolytes are useful and, from the standpoint of improvement of conductivity, are advantageously employed. Salts which can be added to increase the electrical conductivity of the electrolyte include soluble alkali metal, and alkaline earth metal sulfates, sulfides, chromates, phosphates and carbonates, such as sodium sulfate, potassium sulfate, lithium sulfate, calcium sulfate, sodium sulfide, potassium sulfide, lithium sulfide, sodium nitrate, calcium nitrate, potassium nitrate, lithium nitrate, sodium chromate, potassium chromate, potassium dichromate, calcium chromate, sodium orthophosphate, sodium pyrophosphate, potassium carbonate, sodium carbonate, lithium carbonate, etc. The promoters of conductivity can be used in amounts from about 25 weight percent or more of the electrolyte solution. In the preferred aqueous electrolyte solutions, the metal halide content can vary from dilute to saturated solutions which are charged to the cell at a rate of from about 10 to about 1500 cubic centimeters per minute per square foot of apparent electrode surface. Most preferably the electrolyte is present in aqueous solu tion in a concentration of between about 2 and about 25 weight percent.

The preferred organic reactant of the present invention is an olefinic compound, that is, a compound having at least one ethylenically unsaturated carbon-to-carbon double bond which is the reactive site at which the oxygen linkage is formed during the process. Included within the scope of the invention is the use of the unsubstituted and aryl and/ or halogen substituted acyclic and alicyclic monoolefins and polyolefins including straight and branched chain olefins, as well as those in which the ethylenic double bond is in the terminal or non-terminal position or within a cycloaliphatic ring. The olefin may be normally gaseous or liquid. Olefins may be diluted with any suitable inert solvent such as a paraffinic or aromatic hydrocarbon or mixtures thereof including petroleum fractions such as kerosene, etc. Other liquid diluents include cyclohexane, toluene, benzene, xylene, hexane, heptane, iso-octane and those mentioned in U.S. Pat. No.

Typical examples of suitable olefins for use in the preferred process of this invention are the alkenes of the homologous series C,,H wherein n is an integer from 2 to 12, such as ethylene, propylene, butene, pentene, hexene, heptene, dodecene, etc., including olefins in which the double bond is in a non-terminal position, such as Z-buteue, 2-pentene, etc., and branched olefins, such as isobutene, isopentene, 4-ethyl-2-hexene, as well as branched compounds in which the double bond is in the side chain, Such as 2-methylene pentane and alkenyl compounds, such as 4 propene 4' yl octane and cyclic olefins, such as cyclopentene, cyclohexene, cyclooctene, cyclononene, etc. Polyolefins may also be reacted inthe electrochemical reaction of the present invention. Suitable polyolefins include those containing isolated, cumulative or conjugated double bonds, such as diallyl, allene, butadiene, isoprene, 2,3 dimethylbutadiene, etc. As pointed out above, olefins substituted with aryl and halogen groups such as, for example, styrene, stilbene, allyl chloride, chloropropene, vinyl chloride, vinyl bromide, etc. may also be used as the olefin reactant in the present process.

The olefin need not be pure and may contain paraffinic or other impurities or diluents normally found in commercially available olefins. For example, commercial grades of ethylene and propylene are suitable and normally contain low molecular weight paraflins, such as ethane, propane, etc. When a low molecular weight olefin is reacted, a gaseous diluent, such as nitrogen, methane, ethane, propane, etc. may be admixed with the olefin and may be used in amounts between 5 and about volume percent of the total feed.

The throughput of olefin through the anode compart ment or a reaction space outside of the electrochemical system may be selected in such way, for example, that approximately 5 to 95 percent is converted per single pass. It has been shown to be an especially favorable technique to introduce into the anode compartment or conversion chamber a gaseous mixture of the olefin to be converted and an inert gas, the concentration of olefin in the mixture amounting, for example, to 25 to 65 volume percent, preferably 35 to 55 volume percent and to convert per single pass of the gaseous mixture through the anode compartrnent or conversion chamber, 75 to 95 percent, preferably 80 to 90 percent of the introduced olefin. As inert gas, the gaseous paraflins corresponding to the olefin used are especially suitable.

In the operation of the electrochemical process of this invention, the electrolyte solution is passed into the electrochemical cell containing the anode and the cathode. The solution generated in the vicinity of the anode by reaction of the electrolyte, preferably a metal halide in aqueous solution, is termed the anolyte and in the process where olefin is reacted, is that portion of the aqueous medium in which the halohydrin derivative of the olefin is formed. The reaction for the formation of halohydrin is believed to proceed by the following mechanisms:

2X electrical energy 2e- X:

Other theories and mechanisms proceeding via formation of a free radical may be involved in the reactions taking place within the electrolytic cell. The overall reaction which takes place in the anolyte is expressed by the following equation:

3=( J 11,0 J H+ X- 2 X OH in the preferred process where the halohydrin is formed in the anolyte, water is simultaneously electrolyzed at the cathode of the electrolytic cell forming hydroxyl ion and hydrogen. The cathodic electrolyte or catholyte is, therefore, alkaline; and, as the halohydrin intermediate comes into contact with the alkaline catholyte, it is dehydrohalogenated forming olefin oxide, water and regenerated halide ion. In addition, the hydrohalic acid (HX) formed in the anolyte in accordance with Equation 6 above, reacts with the alkaline medium thereby converting that acid to additional metal halide electrolyte. In this process, the reactions which occur in the catholyte are shown by the following equations:

2H2O 20' H: 20H (7) Ex MOH MX H (9) wherein M is, for example, metal such as sodium or potassium and X is halogen, for example, chlorine or bromine.

As pointed out above, the formation of halohydrin can be made to take place in a chamber outside or separate from the electrochemical reaction occurring in the electrolytic cell. This is accomplished by introducing olefin into a reaction zone wherein the anolyte solution from the electrolytic cell is circulated. The halohydrin formed in the olefin reaction zone can be separated therefrom by distillation or can then be passed into the electrolytic cell in the anolyte solution. In the latter case, the halohydrin dissolved in anolyte passes from the vicinity of the anode to the cathode where the above reactions 7 through 9 for the formation of olefin oxide take place and the catholyte solution containing dissolved olefin oxide is removed from the cell for recovery of product. Alternatively, when the halohydrin is separated from the anolyte solution and the olefin reaction chamber, it can be passed directly into the catholyte solution contained within the cell or it can be passed to a separate dehydrohalogenation zone to which catholyte is circulated after being removed from the cell. Still another method of contacting reactants involves withdrawing the anolyte from the olefin reaction zone and passing this solution directly into the cathode zone or into a separate dehydrohalogenation zone. Many other variations of these methods will be apparent from the teachings of this disclosure, and it is to be understood that any combination or arrangement for reacting organic compounds inside or outside of the electrolytic cell or with auxiliary electrolytic cells may be employed in the system of the present invention.

When the olefin is reacted within a partitioned or diaphragmed electrolytic cell to form halohydrin, a gaseous efi'luent can be withdrawn from the anode section of the cell which contains unreacted olefin and dihaloparaffinic derivative as a by-product of the process. When the halohydrin is reacted within the electrolytic cell a gaseous efiluent containing a portion of the olefin oxide product and hydrogen can be directly recovered from the cathode section of the cell.

After the olefin oxide is formed in the catholyte, the resulting electrolyte solution is passed into a separation or stripping zone wherein dissolved oxide product is separated from the electrolyte by distillation and/ or by means of a stripping gas, such as nitrogen, steam, methane, ethane, etc. or any other gas which is inert to the oxidation product. The olefin oxide can then be subjected to further purification, if required, and recovered as a product of the process.

The resulting electrolyte solution separated from the oxide product, contains contaminants which form a tarry or tacky coating on the anode of the electrolytic cell. In cases where a diaphragm is employed, the contaminants often cause plugging of the pores so that efficiency of the process is greatly reduced. These contaminants are cumulative so that when the process is operated in a continuous manner for several hours, a coating is built up on the electrode, usually the anode, and/or walls of the anode chamber. Higher operating voltage and reduced product yield are effects of contamination. Formerly, in continuous operation, it has been necessary to replace the anode or to dismantle and clean the apparatus when conversion falls below the desired level or when the required operating voltage exceeds the practical limit. Although the electrolytic reaction zone can be operated over a wide range of current density, such as between about 20 and about 1500 amperes per square foot of apparent electrode surface, current densities in the upper portion of this range or higher are not desirable from an economic standpoint. The process of the present invention provides a remedy which is commercially feasible and economically practicable inthe elimination of impurities which cause electrode coating and which necessitate higher voltage in the operations of the cell. Also, the present process provides for a uniform production of desired product over long periods of continuous operation.

The contaminants which lead to fouling in the electrolytic cell can be present in very small amounts and can be difiicult to separate and identify as individual compounds. It is postulated without limiting the scope of this invention that at least one of these contaminants is a hydrophobic, tarry condensation product, such as an acetol tar, which is substantially insoluble in the electrolyte solution and which contains a carbon skeleton of six carbon atoms or more. This condensation product may comprise a major or a minor portion of the contaminant mixture which causes fouling in the cell. Contaminants which may be present also include polymeric condensation products, such as for example, hydrogenand carbon-containing dimers. The electrolytic reaction zone may be operated over a wide range of current density such as between 20 and about 1500 amperes per, square foot of apparent electrode surface. The operating voltage of the cell is at least the voltage required to obtain electrolysis of the metal halide. The minimum voltage, therefore, dependsupon the particular electrolytic system. For example, when sodium chloride is used as the source of halogen for the halohydrin intermediate, a voltage of at least 2.2 volts is required, assuming unit activities and standard conditions. Usually the voltage applied is within the range of between about 3 and about 7 volts. In operation, however, the voltage demand is increased above the minimum or decomposition voltage due to a combination of a variety of factors such as, for example activity and over-voltage. It has been found that the olefin reactant introduced to the vicinity of the anode reduces the electrode overvoltage depending upon the particular anode material. In this connection, porous, hollow anodes instead of solid electrodes can be used to introduce the olefin to the anode in a way such that the olefin reactant diffuses through the pores where contact between the olefin, electrolyte and electrode occurs thereby producing product, reducing overvoltage and polarization. Theoretically, during operation, the current density can be varied within a desired range or the current may be reversed to minimize polarization.

The electrolytic cell of the present invention can be used in combination with a hydrogen-oxygen fuel cell to generate electrical energy as at least a partial source of power to the electrolytic cell. In this manner, electrical energy expended during the operation of the electrolytic cell can be recovered and the power required from an external source reduced.

The electrolytic cell can be operated over a relatively wide range of temperatures and pressures, i.e. from about C. to the boiling point of the aqueous electrolyte which, at atmospheric pressure, is usually about 105 C. A pressure within the range of subatmospheric, for example about 0 p.s.i.g., to 300 p.s.i.g. or more can be employed. The temperature and pressure are inter-related to the extent that they are controlled to maintain the aqueous electrolyte system in the liquid phase. Thus, when the cell is operated at a temperature above the atmospheric boiling point of the aqueous electrolyte system, the cell is operated at a pressure sufficiently high to maintain the liquid phase.

According to the present invention, the electrolyte solution from which olefin oxide product has been separated is passed to a treating zone wherein the solution is contacted with an inorganic oxidizing agent in a volume ratio of at least 0.01:1 oxidant to electrolyte. The number of equivalents of inorganic oxidizing agent added is at least one one hundredth the number of equivalents of halide lost from the system in by-product. It has been found that between about 0.1 and about weight percent of oxidant with respect to electrolyte solution is useful in the present process. In the case of halogen or halogen-forming agents, the amount of. oxidizing agent employed is greater than the amount which would bring the pH of the electrolyte solution to a value lower than 12.5. It is most advantageous to employ a ratio of from about 0.25 to about 5, particularly from about 0.5 to about .2.5 moles of oxidizing agent to 100 Faradays of current, electrochemically efl ective in the process. When molecular halogen is used as the oxidizing agent, it is preferred to add halogen in an amount equivalent to the amount removed from the system in the halogenated organic. by-products produced at the anode. In this way a substantially uniform pH can be maintained in the system.

The conditions of operating the treating zone include a temperature of between about C. and about 105 C. and a pressure of from about 15 p.s.i.g. to about 300 p.s.i.g., preferably a temperature and pressure approximating the conditions employed in the cell. The inorganic oxidizing agent used in the treatment may be molecular halogen, an alkali or alkaline earth metal hypohalite, hydrogen peroxide or ozone-enriched air or any combination of these oxidants, although the preferred oxidizing agent is molecular halogen corresponding to the halogen of the metal halide in the electrolyte. Since the metal halide is most preferably a metal chloride or metal bromide, the oxidizing agent is most preferably chlorine or bromineTIhe halogen oxidizing gas may be supplied from an auxiliary electrolytic cell, if desired and the contaminated electrolyte treated inside or outside of the auxiliary cell with the halogen gas generated therein. The halogen oxidizing gas accomplishes a dual function: it adjusts the pH of the electrolyte solution to a more acidic level while simultaneously converting the objectionable contaminants to an innocuous form or an insoluble form which can be more easily separated from the electrolyte solution.

The inorganic oxidizing agent may be used in a concentrated or in a diluted state. Dilution is particularly beneficial when extended surface contact or when a change of residence time of electrolyte in the treating zone is desired. Suitable diluents for the inorganic treating agent include nitrogen, air, water, electrolyte, an unsubstituted or halogen substituted parafiin. Other compounds which are inert to the components in the system may also be employed, if desired.

.. To provide a uniform mixture of the oxidizing agent with the electrolyte solution, backmixing, countercurrent contacting or multipoint injection of the oxidant in a packed, mixed or sparged contacting zone such as a tower, a cyclone or a stirred vessel can be employed. .Also, the oxidant may be injected into the electrolyte or the electrolyte may be sprayed into an atmosphere of an oxidizing gas. The contacting tower may additionally contain perforated trays or bubble caps although expensive contacting equipment is not required for operability.

During treatment with the inorganic oxidant, the pH of the electrolyte'solution is maintained below 12.5; and under preferred conditions the pH of the solution is adjusted within the range of from 7 to 10.5. The duration of treatment usually between about 5 minutes and about 2 hours, depends largely upon the efliciency of mixing and the quantity and type of contaminant present.

(After a suitable period of treatment with one or more of the oxidizing agents, the electrolyte solution is withdrawn from the contacting zone and passed to a separation zone wherein insoluble products and contaminants are removed. Any method for removal of finely dispersed particles or separation of emulsions or dispersions can be employed. For example, the solution can be centrifuged until solids are separated or the solution can be passed through a molecular sieve, asbestos or thermoplastic fiber for physical separation of insoluble materials or the solution can 'be passed through an absorbent, such as carbon, bauxite, bentonite clay, or kieselguhr. Usually, the entire electrolyte solution is subjected to separation, although, in cases where the contaminant level is extremely low, separation can be applied to a portion of the solution, e.g., a slip stream from the solution returned to the cell.

Centrifuging of the solution is usually employed where the insoluble contaminant level is relatively high, whereas sorption or physical separation is usually employed where the insoluble contaminant level is low, for example, less than 0.1 weight percent of the solution. Suitable molecular sieves are those havinglarge pore size to accommodate the relatively large molecular structure of the impurities. Of the filter and absorbent materials which can be used advantageously in this separation step, bentonite caly, diatomaceous earth, kieselguhr, activated carbon, charcoal particles, cellulosic materials, such as wood or paper fibers, inert sand or any of the diaphragm materials are representative of materials recommended. In fact, any absorbent material suitable for the removal of the tarry or tacky condensation products which is inert to the metal halide can be employed, the filter or adsorbent materials are preferably used in a finely divided state. However, when absorbent activityis also a property of the material, a comparatively large particle size can be employed. Generally, the particle size employed is between about and Y325 mesh. The types of filtering apparatus which can be suitably employed in the present process include a filter press, a leaf filter, a rotary drum filter, a pressure filter, a sand filter, a centrifuge filter, etc. although, any other efiicient filtering device can be employed, if desired.

It is to be understood that the oxidizing agent treating and separation steps can be carried out in one or a plurality of stages or a plurality of oxidant treating steps with interposed separation steps can be employed. Also, any combination of oxidizing agents in the same or in individual treating zones as well as any combination of separation techniques may be employed in the present process. The temperature and pressure of the separation treatment is preferably that temperature and pressure employed in the electrolytic cell. However, the solution can be pressurized into the filter or adsorption zone to compensate for pressure drop resulting from passage of the electrolyte through the filter, usually at a flux rate of from about 0.1 to about 100 gallons per square foot of filter per hour. After the separation treatment, or final stage of the separation treatment, the electrolyte solution is ultimately recycled to the electrolytic cell where it is reused without the difliculties caused by contamination.

The following examples are provided for a better understanding of this invention and are not to be construed as unnecessarily limiting to the scope of the invention as set forth above.

EXAMPLE I The electrolytic cell employed in this example is a diaphragmless cell constructed of polyethylene. Positioned within the cell are a porous anode composed of dense graphite and a cathode in the form of a stainless steel screen. In carrying out this experiment aqueous sodium chloride is fed from an-electrolyte reservoir through a flow meter and preheater and is charged to the bottom of the electrolytic cell in the vicinity of the anode with upward flow through a distributing means, for example a frit (sintered structure). Ethylene is employed as the olefin reactant and is fed to the cell in the presence of nitrogen as a diluent gas,.the gaseous mixture being introduced through the back of the porous anode. Eflluent gases from the anode section of the cell are separated from theaqueous sodium chloride solution in a gas-liquid separator positioned at a given height to maintain constant pressure on the cell. The gaseous stream from the anode is passed through a caustic soda scrubber to remove any chlorine, a cold trap to condense dichlorethylene, a gas sampling tube and a wet test meter. The aqueous sodium chloride solution is discharged into a collecting reservoir at the end of the experiment. Gaseous efiluent from the catholyte is passed through a series of scrubbers containing 0.1 normal hydrochloric acid saturated with magnesium chloride to remove any ethylene oxide product, a Dry Ice trap to condense by-products, a gas sampling tube and awet test meter. During the course of this experiment, the electrolyte is analyzed periodically to measure the concentration of the oxide and to record the pH. The conditions under which this experiment is conducted include: an ethylene flow rate of 80 cc. per minute, an ethylene: nitrogen mole ratio of 1:1; the use of aqueous sodium chloride solution containing 131 grams per liter of sodium chloride introduced to the cell at a rate of 77 cc. per minute, and a pH of 10.6 for this aqueous feed solution. The cell is operated at a temperature of 40 C. at a current density of 100 amperes per square foot of apparent electrode surface. The pH of the anodic electrolyte at the end of the run is 7.7, and that of the exit brine solution from the region of the cathode'is 11.7. It is found that no chlorine exits from the vicinity of the anode, the gaseous efiluent from the anode consisting essentially of unreacted ethylene.

The chlorine efiiciency of the cell is about '97 percent, and the hydrogen efficiency is about 99 percent, 0.185 mole of hydrogen being produced. Under these conditions the total organic product yield is 25.7 percent, of which about 25.2 percent is ethylene oxide. Partial work-up of the electrolyte by simple fractional distillation at 25-27 inches mercury pressure and about 40 to 45 C. yields 48 percent ethylene oxide, the remainder of the organic product being predominantly oxide (about 50 percent) and containing a small amount of dichloroethylene (about 2 percent).

The product and by-product are separately recovered from upper portions of the distillation zone. A bottoms liquid electrolyte solution fraction is withdrawn from the distillation zone at the rate of about 75 cc. per minute. The pH of the electrolyte solution is about 12. The electrolyte solution is then passed into the upper portion of a sparged tower for countercurrent contact with molecular chlorine entering the lower portion of the tower at a rate sufiicient to adjust the pH of the electrolyte solution to 10. The electrolyte is contacted with the molecular chlorine for a period of about 20 minutes after which it is removed and passed to a separation zone wherein it is c'en"- trifuged at 1000 times gravity for a period of 10 minutes. The supernatant liquid which is primarily a decontaminated aqueous solution of sodium chloride is then recycled to the bottom of the electrolytic cell. The trace amount of solids separated in the centrifuge zone is discarded and the electrochemical process is carried on in a continuous manner. After 8 months of continuous operation, the anode and walls of the cell are completely free of coating and the yield of product unreduced.

Any of the olefins described in the preceding disclosure can be substituted for ethylene in the above example to provide at least equally good results for continuous operation. For example, when allyl chloride is substituted for ethylene the only change in the example relates to the olefin feed and the products produced. Thus, trichloropropane, a dichloropropanol and epichlorohydrin are products of the reaction.

EXAMPLE II The electrolytic cell employed in this example is a diaphragm cell using an aqueous solution of potassium chloride having a pH of about 10 as the feed electrolyte. The diaphragm separating the anode chamber from the cathode chamber is composed of polyethylene fibers having a thickness of 0.025 inch. Vertically disposed in the cell is a porous carbon anode and a stainless steel screen is the cathode having an apparent area of about 0.17 square foot.

The olefin reaction zone consists of a vertically arranged conduit about 30 mm. in diameter provided at its base with a titanium metal frit. This reaction zone is connected with the anode compartment by two horizontally extending pipes, one terminating above the other below the anode. A gas separator through which a gaseous effluent can be removed is arranged between the olefin reaction zone and the upper connecting pipe to the anode compartment. The electrolytic cell and the olefin reaction zone are filled with '5 percent aqueous potassium chloride solution of which 4 liters per hour are delivered from the anode compartment through the diaparagm and into the cathode compartment. The chlorohydrin which forms in the electrolyte is dehydrochlorinated in the cathode compartment to form the corresponding olefin oxide. A quantity of the catholyte corresponding to the input of anolyte is removed from the cathode compartment and the olefin oxide present in the aqueous mixture together with other oxygenated by-products of the reaction, is separated from the electrolyte in a series of flashing zones. The temperature of the electrolyte in the cell and in the olefin reaction zone is maintained at about 50 C. and a pressure of about 15 p.s.i.g. is imposed on the system.

About 45 liters per hours of a C fraction containing percent by weight ethylene is introduced through the frit into the electrolyte contained in the olefin zone so that the gas rises upwards in a state of finely distributed gas bubbles. Electrolyte circulation is produced through the connecting pipes between the gas containing electrolyte in the olefin reaction zone and the substantially gas-free electrolyte in the anode compartment of the cell. A potential of 3.54 volts is applied across the electrodes of the cell so that a direct current with a current density of 116 amperes per square foot of apparent electrode surface flows over the period of 4 hours.

The volatile gaseous reaction product comprises a mixture of dichloroethane and unrecated olefin which is separated from the anolyte containing ethylenechlorohydrin and dichloroethane is recovered as a product of the process. The anolyte containing ethylenechlorohydrin is passed through a diaphragm and the chlorohydrin is converted to ethylene oxide in the vicinity of the cathode. A gaseous etfiucnt of ethylene oxide and hydrogen is withdrawn from the cathode chamber and the components recovered as products of the process. Liquid catholyte containing dissolved ethylene oxide is also removed from the cathode chamber at a rate of 4 litres per hour and passed to a series of flashing zones wherein at a temperature of 55 C. .under 2 p.s.i.g. vaporous ethylene oxide and by-products of the reaction are separated from the liquid electrolyte solution. The vaporous fraction from the flashing zone is purified and ethylene oxide recovered as a product of the process.

The liquid electrolyte is withdrawn from the final flashing zone and is passed to a contacting vessel for contact with potassium hypochlorite which enters as an aqueous solution. After about 40 minutes contact, the electrolyte solution containing predominantly potassium chloride as the major component of the aqueous solution is treated with hydrogen chloride to adjust the pH of the solution to 9.5 and the resulting solution is then passed to a filtra tion zone containing activated carbon. Two liters of electrolyte solution per square foot of filter per hour is passed through the filter and the resulting electrolyte solution substantially free of coating contaminants is recycled to the anode chamber. of the electrolytic cell which continues to operate for a period of more than 10 months.

As an alternative to the above decontamination steps, the liquid electrolyte, after treatment with potassium hypochlorite, can be passed into a halogen treating tower wherein chlorine gas is added to adjust the pH of the solution to approximately a pH of 10. The resulting chlorine treated electrolyte solution can then be filtered or centrifuged in one or more stages before recycle to the anode portion of the electrolytic cell. Another variation of the above decontamination process comprises treating the electrolyte solution in the above example with chlorine to a pH. of 10 after the solution is withdrawn from the filtration zone containing activated carbon. In this case, the elec trolyte solution is again filtered after leaving the chlorine treating zone. These and many other modifications of the above process, e.g., any of the electrodes, electrolyte cells, filtering means and apparatus and oxidizing gases mentioned above, can be substituted for high performance of the electrochemical process under continuous operation.

EXAMPLE III In this example, a glass electrolytic cell having a platinum anode and a cathode comprising a pool of mercury 1 inch deep in contact with a platinum wire is used. The cell is charged with a 10 percent aqueous solution of sodium bromide as the electrolyte and propylene is bubbled through the solution for conversion to propylene oxide. The ofi gas is collected in a Dry Ice trap while 5 volts is imposed across the electrodes causing a current of about 1 ampere to flow through the electrolyte solution. The current flow is maintained for 13 hours and after about 12 hours the pH is increased from an initial pH- of 7 to an end pH of 9. The solution is maintained at about 50 C. to 55 C. under atmospheric pressure conditions. The electrolyte 12 solution is then separated from the oxidation reaction product by distillation and the oxidation product is recovered as 97 percent propylene oxide.

A liquid electrolyte is separately withdrawn from the distillation zone and is passed into a mixing vessel wherein it is contacted with molecular bromine which is jet injected into the mixing vessel at a rate and in an amount sufficient to lower the pH of the electrolyte solution to 7. The electrolyte is treated with bromine for a period of 15 minutes at 55 C. under atmospheric pressure after which the solution is pumped to a filtration chamber wherein it is passed through a filter zone comprising finely divided particles of diatomaceous earth (average particle size of 325 mesh). The filtered electrolyte solution is then recycled to the vicinity of the anode in the electrolytic cell and the above process is repeated in the manner of continuous operation. After 6 months of operation no coating is found on the anode or other portions of the cell, the product yield is continued at substantially high levels and substantially no increase in voltage drop is evidence between the electrodes of the cell so that the highly efficient continuous operating conditions are maintained.

EXAMPLES 4 THROUGH 15 The following examples are presented for the purposes of comparing an electrolytic process wherein the electrolyte is recycled without decontamination and where the electrolyte is recycled after the decontamination treatment of the present invention. In Examples 4 through 6 inclusive and 10 and 11 inclusive, propylene is converted to propylene chlorohydrin within an electrolytic cell'containing a diaphragm. In the remainingexamples propylene is converted to chlorohydrin in an external olefin reactor wherein the propylene is contacted with anolyte circulated from the anode chamber to an electrolytic cell containing a diaphragm.

The diaphragm employed in the electrolytic cell of Examples 4 through 9 and 12 is an acrylonitrile diaphragm of 0.02 inch thickness. The remaining examples employ an asbestos diaphragm of 0.04 inch thickness. In Examples 4, 5, 6 and 8, the electrolyte is an aqueous solution of 5 weight percent potassium chloride, in all other examples the electrolyte is 8.5 weight percent potassium chloride. The flow rate of electrolyte in Examples 4 through 8 is 4 kilograms per hour; in Examples 10 and 11, it is 12 kilograms per hour and in the remaining examples, it is 6 kilograms per hour. Examples 4 through 6 employ 70 percent platinum/ 30 percent iridium coated on a titanium sheet as the anode of the cell. The remaining examples employ a ruthenium oxide coated titanium sheet as the anode. The cathode in all cases is a stainless steel screen.

and the electrolyte reaction in all examples is carried out at a cell temperature of 50 C. under a pressure of 15 p.s.i.g. The results of these experiments are presented in following Table I as Examples 4 through 15, inclusive.

From the data presented in Table I, it is evident that decontamination of the recycle electrolyte is necessary in avoiding anode coating. Although comparison of Examples 4 through 9 with 10 through 15 shows little difference in the results obtained for increase in the anode voltage and increase in cell voltage over the periods recorded, it should be noted that Examples 4 through 9 measure relatively short periods; whereas 10 through 15 measure much longer periods of time. As pointed out hereinabove, the contaminants in the untreated recycle electrolyte are cumulative and thus Examples 4 through 9 would show larger increases in anode and cell voltage had they been measured for the same duration of time as Examples 10 and through 15. The same explanation applies to the change in propylene oxide yield for Ex,- amples 4 through 9 as compared to Examples 10 through 13 where the yield is measured.

In the Examples 4 through 15 the decontaminationtreatment consists of contacting the electrolyte solution after the removal of propylene oxide with molecular 13 chlorine at a rate of 1 or 2 grams per hour at 50 C. under atmospheric pressure. The pH of the solution is thus adjusted to 10. After about 20 minutes of treatment the electrolyte solution is withdrawn from the treatment zone and passed to a filtration tower containing relatively large particles of carbon (i.e., average particle size of 80 mesh). The filtered electrolyte solution is then returned to the system in the vicinity of the anode. Much smaller variations in the anode voltage and cell voltage and product yield can be realized when using carbon particles of a smaller size, for example, average particle size between about 150 mesh and about 325 mesh.

14 The reaction products contained in the anode and cathode exit gases and the catholyte are analyzed and found to be as follows:

Product yield in percent Carbon dioxide TABLE I.-CONTINUOUS ELECTROCHEMICAL PROCESS FOR PRODUC- TION OF PROPYLENE OXIDE CHLORIDE ELECTROLYTE WITH RECYCLE OF AQUEO US POTASSIUM' Current density, A yield I amps/it. CzHuO, A P Hours eat. surface A anode percent A cell anolyte Coating on Examples duration anode voltage Faraday voltage (cm.) anode 185 Yes. 166 163 .6-5.3 258 1. 4-1.7 .3-4.6 138 1. 7-1. 9 5-5. 2 90 1. 6-1. 7 .7-5. 2 627 1. 5-1. 6 8-6. 1 434 1. 6-1. 8 9-6. 9 622 1. 4-1. 7 2-5. 8 1, 147 1.6-1.8 2-4. 6

EXAMPLE 16 In this example, an electrolytic cell is employed wherein a solid sheet anode and a wire mesh cathode are disposed in a position opposite and parallel with respect to one another. Each of the electrodes are 100 mm. Wide and 750 cc. high. The anode is composed of a platinum coated titanium metal sheet and the cathode of stainless steel. A diaphragm comprising an asbestos web located between the electrodes is attached to the cathode screen. The distance between the anode and the diaphragm facing the anode is 6 mm.

After the cell 'is filled with an 8.5 percent aqueous potassium chloride solution, a direct current is passed between the anode and the cathode. The cell voltage is 4.4 volts at a current density of 12.3 amperes per dm. of anode surface. The temperature of the electrolyte is 59 C. and the cell is operated under atmospheric pressure. About 95 liters per hour of a C -hydrocarbon fraction containing 92 weight percent ethylene (the remainder being mostly ethane), are introduced into the anode zone in a finely dispersed state by means of a sparge located anolyte and removed from the anode zone of the cell.

About 9.5 liters per hour of electrolyte are passed from the anode zone through the diaphragm to the cathode zone where the ethylene chlorohydrin is converted to ethylene oxide.

The catholyte containing product is removed from the cathode zone at a rate corresponding to the anolyte feed rate and the ethylene oxide is then separated from the electrolyte by distillation. The resulting olefin oxide-free electrolyte is subsequently passed at a temperature of about 55 C. through the anode zone of an auxiliary electrolytic cell which is connected into the electrolyte recycle line. This auxiliary cell is operated at a current intensity of 1.3 amperes, generating about 1.7 grams of elemental chlorine in a finely dispersed state into the recycle electrolyte. The residence time in the cell is about 40 minutes. The treated electrolyte is then passed through a filtration zone containing finely divided diatomaceous earth in the form of a course powder. After adjusting the pH to 9 or 10 by the addition of hydrogen chloride, the filtered electrolyte is then returned to the anode zone where the reaction of ethylene takes place.

The system is operated in a continuous manner for about 12 months without showing any signs of electrolyte contamination. During this time, no coating of the anode is observed.

The above process is repeated except that treatment of recycle electrolyte with chlorine and filtration of the treated recycle is omitted. Contamination of recycle electrolyte occurs after only two days of operation. Strongly colored organic compounds are present in the electrolyte which cause an increase in the cell potential and which finally lead to deposits on the electrodes necessitating interruption of the process.

EXAMPLE 17 In the following example, the apparatus comprises an electrolytic cell and a separate reaction zone for the conversion of olefin to the corresponding chlorohydrin. The electrolytic cell contains an anode spaced 4.5 mm. from a cathode, the electrodes being parallelly disposed and each having a surface area of 1.7 dm. A woven polyethylene fiber diaphragm 0.4 mm. thick is interposed between the electrodes and contacts the cathode surface thereby dividing the cell into an anode chamber and a cathode chamber. The anode is composed of a solid sheet of titanium coated with a layer of noble metal oxide, in this case ruthenium oxide, on the side which faces the cathode. The cathode is a stainless steel wire mesh.

The olefin reaction zone comprises a vertically disposed pipe of 26 mm. diameter and is equipped in its lower portion with a glass sparge. The reaction zone is connected with the anode zone of the electrolytic cell by means of two horizontal pipes, one of which is terminated above the other below the anode. A gas trap is placed between the reaction zone and the upper pipe connected to the anode zone. A gas trap separator is employed to remove gases contained in the electrolyte.

The electrolytic cell and the olefin reaction zone are filled with 8.5 percent aqueous potassium chloride solution. A current is applied to the cell. The current density at the anode is 32.8 amperes per dm. and the cell voltage is 4.9 volts.

Into the cell, operated at 59 C. under atmospheric pressure, is introduced 61 liters of a c -hydrocarbon fraction containing 94 weight percent propylene, the remainder being mostly propane. The organic reaction mixture is bubbled upwardly through the glass frit in a finely dispersed state. About 38 percent of the propylene is converted to propylene chlorohydrin and the remaining gas 1s removed from the gas space above the level of the electrolyte and from the olefin reaction zone. Intensive circulation of the electrolyte between the reaction zone and the anode zone is maintained. Gas rich electrolyte rises in the reaction zone while substantially gas free electrolyte moves downwardly in the anode zone between the anode and the diaphragm and 6 liters per hour of the electrolyte solution are transferred from the anode zone to the cathode zone. The propylene chlorohydrin contained in the anolyte is dehydrohalogenated to propylene oxide in the cathode zonewhile an amount of catholyte, corresponding to the amount of anolyte added, is removed from the cell and propylene oxide is separated therefrom by distillation.

The electrolyte stripped of propylene oxide is passed upwardly through a 400 cm. tower where it is contacted with 2 grams per hour of elemental chlorine at about 55 C. The residence time in the tower is about 30 minutes. The pH of the treated electrolyte is adjusted to about 9.5 by the addition of hydrogen chloride. The chlorine treated electrolyte is passed through a filtration zone containing activated carbon of average size 400 mesh. The treated electrolyte is then returned to the anode zone of the electrolyte cell.

The gaseous and liquid products withdrawn from the cell are analyzed and the results reported below.

Product yield in percent After 8 months of continuous operation no coating on the anode or other indications of contamination are noted, and the system is operated continuously for a period of 12 months without showing any signs of contamination.

The above example is repeated except that the treatment of propylene oxide free electrolyte with elemental chlorine and subseuent filtration is omitted. Contamination of the recycle electrolyte occurs within two days. Strongly colored organic compounds are concentrated in the electrolyte which cause an increase in the cell potential and deposits on the anode so that it is necessary to interrupt the process.

EXAMPLE 18 The arrangement of apparatus described in Example 17 is again employed for this example except that an asbestos paper web diaphragm, 1.2 mm. thick, is substituted for the polyethylene diaphragm of the preceding example and an external dehydrochlorination column, 1200 mm. high and 25 mm. in diameter, packed with 4 mm. glass Rashig rings is added to the apparatus arrangement. The column with a heated sump is operated under mm. Hg and is equipped with a receiver, cooled by true.

The electrolytic cell and the olefin reaction zone under atmospheric pressure are filled with a 5 percent aqueous potassium chloride solution. Of this solution two equal portions are then each pumped at a rate of 2 liters per hour from the upper pipe line between the olefin reaction zone and the anode zone and from the cathode zone respectively into the mid-section of the column. The electrolyte solution in equal amounts is returned to the anode and cathode zones respectively of the cell after separation of volatile products which are vaporized in the column.

The electrolyte. stripped of propylene oxide is subjected to the chlorine treatment at a temperature of about 55 C. by passing it upwardly through a tower having a capacity of 250 cmfi. The chlorine is passed in a finely dispersed state at a rate of 0.5 gram per hour into the bottom of the tower. After repeating the filtration'step of Example 17, the electrolyte is then removed from the filtration zone and hydrogen chloride is added to adjust the pH to about,8.5. The concentration of the potassium chloride in the aqueous recycle solution is controlled by the addition of water and the solution is heated to a temperature such that the cell is maintained at 59 C. a

About 45 liters per hour of aC -hydrocarbon fraction containing 94 weight percent propylene, the remainder being mostly propane, is introduced through the glass frit into the electrolyte upwardly into the reaction zone and unconverted gas in the olefin reaction zone is removed from the gas space above the electrolyte level in the zone. The substantially gas free electrolyte then passes downwardly into the anode zone and a high circulation rate is maintained.

A direct current is applied to the electrodes of the cell at a current density of 11.2 amperes per dm. of anode surface. The cell voltage is 3.45 volts. The gaseous reaction products from the olefin reaction zone and the cathode zone and the liquid reaction products from the receiver of the dehydrochlorination column and the products remaining in the catholyte removed from the cell are analysed and the results are reported as follows:

Product yield in Reaction product: percent Faraday Propylene oxide 89.6 1,2-dichloropropane 8.1 Propanediol-l,2 1.4 Propylene chlorohydrin 0.4 Other organic products 15 Oxygen 0.3 Carbon dioxide 0.5

The system is operated continuously for 8 months during which time no contamination occurred in the system. This example is repeated except that treating the electrolyte, separated from the propylene voxide, with chlorine and filtration before recycling into the electrochemical system, is omitted. Contamination of the recycle electrolyte takes place after only two days. Strongly colored, organic compounds are concentrated in the electrolyte which causean increase of the cell potential and form deposits on the electrodes resulting in interruption of the continuous operation.

It is to be understood that hydrogen peroxide or air enriched with between about 0.01 percent to about 3 percent ozone can be substituted in any one of the preceding examples for the inorganic oxidizing agent to convert the deleterious contaminants of the electrolyte solution. When these oxidants are employed in the treating zone, it may be necessary to subsequently adjust the pH of the electrolyte solution with halogen or hydrogen halide. ,It is also understood that any of the filter or adsorbent materials mentioned in the teachings of the specification may be substituted for the filter materials recited in any of the preceding examples to provide a decontaminated electrolyte solution suitable for recycling to the electrolytic cell in a continuous operation. 3

Having thus described our invention, we claim:

. 1. A process for purifying the electrolyte medium generated in an electrolytic cell wherein an olefinic compound has been electrochemically converted without'a redox agent to form the corresponding 'oxide and said oxide has been separated from said electrolyte medium which comprises:

treating the electrolyte medium in a separate treating zone with at least one inorganic oxidizing agent selected from the group consisting of molecular halogen, a metal hypohalite, hydrogen peroxide, and

17 ozone-enriched air prior to recirculation of the electrolyte medium to the electrolytic cell.

2. A process for purifying the electrolyte medium generated in an electrolytic cell wherein an olefinic compound has been electrolytically converted without a redox agent to form the corresponding oxide and said oxide has been separated from said electrolyte medium which comprises:

treating the electrolyte medium in a separate treating zone with at least one inorganic oxidizing agent selected from the group consisting of molecular halogen, a metal hypohalite, hydrogen peroxide, and ozone enriched air, and

passing said treated electrolyte medium through a separation zone for the removal of contaminants prior to recirculation of the electrolyte medium to the electrolytic cell.

3. The process of claim 1 wherein said oxidizing agent is molecular halogen.

4. The process of claim 2 wherein said oxidizing agent is molecular halogen.

5. The process of claim 1 wherein the pound comprises ethylene.

6. The process of claim 1 wherein the pound comprises propylene.

7. The process of claim 1 wherein the pound comprises allyl chloride.

8. The process of claim 1 wherein the pound comprises butylene.

9. The process of claim 1 wherein the pound comprises styrene.

10. The process of claim 1 wherein the electrolyte medium is an aqueous solution of an alkali metal halide.

11. The process of claim 1 wherein the electrolyte medium is an aqueous solution of an alkaline earth metal halide.

12. The process of claim 1' wherein the electrolyte medium is an aqueous solution of an alkali metal or alkaline earth metal halide containing an inorganic salt for improving the electrical conductivity of the solution, which salt is a sulfate, sulfide, chromate, phosphate or carbonate of an alkali metal or an alkaline earth metal.

13. The process of claim 12 wherein the conductivity promoting salt comprises between about 1 and about 25 weight percent of the electrolyte medium.

14. The process of claim 1 wherein the electrolyte medium is an alkali metal or alkaline earth metal halide having a concentration of from about 2 weight percent to about 25 weight percent in the aqueous medium.

15. The process of claim 1 wherein the electrolyte medium is an aqueous solution of an alkali metal or alkaline earth metal cholride or bromide, the inorganic oxidizing agent is molecular halogen wherein the halogen corresponds to the halogen of the alkali metal halide or alkaline earth metal halide which is used as the electrolyte and the halogen-treated electrolyte solution is passed through a filtration zone for the separation of contaminants.

16. The process of claim 15 wherein the molecular halogen oxidizing agent is obtained from an auxiliary electrolytic cell.

17. The process of claim 16 wherein the electrolyte is treated with the molecular halogen oxidizing agent in the anode chamber of the auxiliary cell.

olefinic comolefinic olefinic comolefinic comolefinic com- 18. The process of claim 15 wherein the electrolyte is countercurrently contacted with molecular halogen oxidizing agent in a tower.

19. The process of claim 1 wherein the electrolyte medium is an aqueous solution of a chloride or a bromide of an alkali metal or alkaline earth metal, the inorganic oxidizing gas agent is a metal hypochlorite or a metal hypobromite corresponding to the halogen of the electrolyte and the halogen-treated electrolyte solution is passed through a filtration zone for the separation of contaminants.

20. The process of claim 1 wherein the inorganic oxidizing agent is hydrogen peroxide.

21. The process of claim 1 wherein the inorganic oxidizing agent is ozone-enriched air.

22. The process of claim 2 wherein the separation of contaminants from the treated electrolyte solution is effected by centrifuging.

23. The process of claim 2 wherein the separation of contaminants from the treated electrolyte solution is effected by means of filtration.

24. The process of claim 23 wherein the filtration medium is a molecular sieve, asbestos activated carbon, charcoal, diatomaceous earth, kieselguhr, bentonite clay, a cellulosic fibrous material, polyacrylonitrile, Teflon, polypropylene or polyethylene.

25. The process of claim 1 wherein the electrolyte solution is contacted with an inorganic oxidizing agent in a ratio of from about 0.25 to about 5, preferably from about 0.5 to about 2.5 mols of oxidant to 100 Faradays of current, electrochemically effective in the process, and Where the treatment with the oxidizing agent is eflected over a period of from about 5 minutes to about 2 hours.

26. The process of claim 25 wherein the inorganic oxidizing agent is diluted with a minor amount of nitrogen, air, or a hydrocarbon paratfin or halogen-substituted paraffin.

27. The process of claim 1 wherein the electrolyte solution is treated in a plurality of treating zones employing the same or different inorganic oxidizing agents and the last in the series of treating zones employs halogen gas wherein the halogen gas corresponds to the halogen of the metal halide electrolyte.

28. The process of claim 27 wherein the separation of contaminants is effected in a plurality of separating steps involving filtration and/or adsorption.

29. The process of claim 28 wherein a separation step follows each of the oxidant treating steps.

References Cited UNITED STATES PATENTS 3,288,692 11/1960 Leduc 204 3,479,262 11/1969 MacLean et a1. 20480 3,397,226 8/1968 Fenton 20480 3,397,225 8/1968 Fenton 20480 FOREIGN PATENTS 3,826 3/ 1886 Great Britain 20479 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

